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Conclusions
This exercise was primarily aimed at building a data migration and testing it against
some real data. Inevitably, this is only a drop in the ocean of the possible testing you
could do for a migration. You could imagine building automated tests to check that all
your data was preserved, comparing the database contents before and after. You could
write individual unit tests for the helper functions in a data migration. You could spend
more time measuring the time taken for migrations, and experiment with ways to speed
it up by, eg, breaking up migrations into more or fewer component steps.
Remember that this should be a relatively rare case. In my experience, I haven’t felt the
need to test 99% of the migrations I’ve worked on. But, should you ever feel the need
on your project, I hope you’ve found a few pointers here to get started with.
On Testing Database Migrations
Be wary of migrations which introduce constraints
99% of migrations happen without a hitch, but be wary of any situations, like this
one, where you are introducing a new constraint on columns that already exist.
Test migrations for speed
Once you have a larger project, you should think about testing how long your
migrations are going to take. Database migrations typically involve downtime, as,
depending on your database, the schema update operation may lock the table it’s
working on until it completes. It’s a good idea to use your staging site to find out
how long a migration will take.
Be extremely careful if using a dump of production data
In order to do so, you’ll want fill your staging site’s database with an amount of data
that’s commensurate to the size of your production data. Explaining how to do that
is outside of the scope of this book, but I will say this: if you’re tempted to just take
a dump of your production database and load it into staging, be
very
careful. Pro‐
duction data contains real customer details, and I’ve personally been responsible
for accidentally sending out a few hundred incorrect invoices after an automated
process on my staging server started processing the copied production data I’d just
loaded into it. Not a fun afternoon.
Conclusions
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